1. Field of the Invention
The present invention relates to a semiconductor device that uses a crystalline semiconductor film obtained through crystallization on an insulating surface by using a laser light and to a manufacturing method thereof.
2. Description of the Related Art
Conventionally, a semiconductor display device as an example of semiconductor devices has a drive circuit formed on a silicon substrate, which is connected to a pixel portion on a glass substrate via an FPC and the like. However, ICs are connected to the glass substrate on which the pixel portion is formed via the FPC and the like, there arises a problem in that a connected portion is weak against any physical impact. In particular, as the number of pins of the FPC increases, there is growing tendency for the connected portion to exhibit a poor property against the physical impact.
Then, techniques of integrating a drive circuit and a controller of the semiconductor display device with the pixel portion on the same glass substrate (system on glass) have been put into active research and development. The realization of the system on glass makes it possible to avoid the aforementioned problem by reducing the number of pins of the FPC as well as to reduce a size of the semiconductor display device itself. Further, since the glass substrate is cheaper than the mono-crystalline silicon substrate, it is possible to reduce manufacturing costs of the semiconductor display device.
For example, in a case of an active matrix liquid crystal display device as an example of the semiconductor display devices, a scanning line drive circuit and a signal line drive circuit are formed on the same glass substrate, the scanning line drive circuit being used for sequentially selecting one or more pixels among the plural pixels formed in the pixel portion and the signal line drive circuit being used for inputting signals (video signals) having image information to the selected pixels. This makes it possible to enhance resistance to the physical impact in the liquid crystal display device and to reduce the size of the liquid crystal display device itself.
Further, in recent years, integral formation of the controller, a CPU and the like, which has been conventionally formed on the silicon substrate, on the glass substrate, is being attempted in addition to the drive circuit. If both of the controller and the drive circuit can be integrally formed on the glass substrate on which the pixel portion is formed, the size of the semiconductor display device can be remarkably reduced and the resistance to the physical impact can be further enhanced.
Further, a glass substrate is inferior in heat resistance and is easily subjected to thermal deformation. Therefore, in the case where a crystalline TFT is formed on the glass substrate, in order to avoid thermal deformation of the glass substrate, the use of laser annealing for crystallization of a semiconductor film is extremely effective. Laser annealing has characteristics such as remarkable reduction of processing time compared to an annealing method utilizing radiant heating or conductive heating. In addition, a semiconductor or a semiconductor film is selectively and locally heated so that a substrate is scarcely thermally damaged.
Note that the term “laser annealing method” herein indicates a technique of recrystallizing a damaged layer formed on a semiconductor substrate or on a semiconductor film, and a technique of crystallizing an amorphous semiconductor film formed on a substrate. This also includes a technique that is applied to leveling or improvement of a surface quality of the semiconductor substrate or the semiconductor film. Applicable laser oscillation apparatuses include: gas laser oscillation apparatuses represented by an excimer laser; and solid laser oscillation apparatuses represented by a YAG laser. It is known that such an apparatus performs laser light irradiation to thereby heat a surface layer of the semiconductor in an extremely short period of time of about several tens of nanoseconds to several tens of microseconds for crystallization.
In general, the crystalline semiconductor films formed by using the laser annealing method are formed of an aggregation of plural crystal grains. The crystal grains develop randomly in position and size thereof. Thus, it is difficult to form the crystalline semiconductor film while designating the position and the size of the crystal grains. As a result, in an active layer formed by patterning the crystalline semiconductor film into an island-like shape, an interface (grain boundary) of the crystal grains may exist.
Note that the term grain boundary, which is also called a crystal grain boundary, refers to one of lattice defects categorized as a plane defect. The plane defect includes not only the grain boundary but also a twin plane, a stacking fault, or the like. In this specification, the plane defects having electrical activity and dangling bonds, i.e., the grain boundary and the stacking fault are collectively called the grain boundary.
Differing from the inside of the crystal grain, the grain boundary includes a number of recombination centers and trapping centers due to an amorphous structure, crystal defect, or the like. When carriers are trapped in the trapping center, a potential of the grain boundary increases, which serves as a barrier against the carriers. As a result, it is known that the carriers are decreased in current transporting characteristics. Thus, for example, in the case where the TFT is formed as a semiconductor device, when the grain boundary exists in the active layer, particularly in a channel formation region, it significantly affects the characteristics of the TFT as follows. That is, mobility of the TFT is remarkably decreased, an ON current is decreased, an OFF current is increased due to the current flowing through the grain boundary, and the like. Also in the plural TFTs manufactured on the assumption that the same characteristics can be obtained, the characteristics may vary depending on whether or not the grain boundary is included in the active layer thereof.
When the laser light is irradiated onto the semiconductor film, the crystal grains are obtained randomly in terms of position and size on the grounds listed below. That is, it takes a certain amount of time for the nucleation of solid phase to develop in the completely melted semiconductor film in a liquid form due to the laser light irradiation. Then, a number of crystal nuclei are generated with time in the completely melted region and crystal growth occurs from each of the crystal nuclei. The crystal nuclei are generated in random positions and thus, distributed in a nonuniform manner. The crystal growth terminates at a position where the crystal grains are abutted against each other, so that the crystal grains develop randomly in position and size.
The transistor used in a drive circuit, a controller and a CPU is required to operate at high speed. As described above, however, it is difficult to form the single crystal silicon film having no grain boundary by the laser annealing method. The TFT using as the active layer the crystalline semiconductor film crystallized by the laser annealing method, which has characteristics equivalent to those of a MOS transistor formed on the single crystal silicon substrate has not been realized so far.
In view of the above-mentioned problems, a second object of the present invention is to provide a manufacturing method for a semiconductor device using a laser crystallization method, which can prevent a grain boundary from developing in a channel formation region of a TFT and avoid a remarkable reduction in mobility of the TFT due to the grain boundary, a decrease in an ON current, and an increase in an OFF current and to provide a semiconductor device manufactured by using the manufacturing method. Moreover, the present invention is to provide a designing method for a semiconductor device using aforementioned crystallization method.
The applicants of the present invention discovered that if a semiconductor film is formed on an insulating film having unevenness, and laser light is irradiated to the semiconductor film, then grain boundaries are selectively formed on portions, which are located on projective portions of the insulating film, of the crystallized semiconductor film.
FIG. 19 shows a TEM image of a test piece, seen from above, when continuous wave laser light having an output energy of 5.5 W is irradiated to a 150 nm thick amorphous semiconductor film, which is formed on a base film having unevenness, along a longitudinal direction of the projective portions with a scanning speed of 50 cm/sec. Further, the TEM image shown in FIG. 20 schematically shows the TEM image shown in FIG. 19 for easier explanation thereof to be understood.
Among semiconductor films in FIG. 19 and FIG. 20, a region denoted by the reference numeral 8001 corresponds to a portion located on a portion of the projective portion, and a region denoted by reference numeral 8002 corresponds to a portion located on a portion of the depressive portion. The term depressive portion indicates a depressed region on which the projective portion is not formed. The width of a projective portion is 0.5 μm, the width of a depressive portion is 0.5 μm, and the thickness of the projective portion is 250 nm. As shown in FIG. 20, a grain boundary 8003 is formed in the semiconductor film on the projective portion.
FIG. 21 shows a TEM image, in its cross section in a direction orthogonal to a laser light scanning direction, of a test piece, which is manufactured under the same conditions as those for the test piece shown in FIG. 19 and then undergoes Secco etching. A base film having unevenness is formed of a three layer insulating film. A second insulating film made from silicon oxide having a stripe pattern is formed on a first insulating film made of silicon nitride, and a third insulating film made of silicon oxide is formed covering the first insulating film and the second insulating film.
Note that Secco etching is performed at room temperature for 75 seconds using an aqueous solution mixed with K2Cr2O7 and HF.
A grain boundary 8005 on a projective portion 8009 is expanded by Secco etching, and its position becomes clearer, as shown in FIG. 21. Note that a white portion visible within the projective portion 8009 shows the region that silicon oxide is etched through the grain boundary due to the semiconductor film grain boundary which is expanded by Secco etching. Also, by irradiating the laser light, the surface of the semiconductor film 8006 is leveled.
Further, FIG. 22 shows a TEM image, which is seen from above, of a test piece which is manufactured by the same conditions as those for the test pieces shown in FIG. 19 and FIG. 21 and undergoes Secco etching. The Secco etching conditions are the same as those of FIG. 21. A region denoted by reference numeral 8501 corresponds to a semiconductor film located on a projective portion, and a region denoted by reference numeral 8502 corresponds to a portion located on a depressive portion. A white portion 8503 visible in the portion located in the upper portion of the projective portion 8501 shows a portion in which a grain boundary of the semiconductor film is etched and expanded by Secco etching, and it becomes clear that the grain boundary is selectively formed on the upper portion of the projective portion 8501.
From this fact, the applicants of the present invention consider that one of the causes of the grain boundary developing on the projective portion is that: volumetric movement of the semiconductor film, which is located on the upper portion of the insulating film, toward the direction of a bottom portion of the depressive portion occurs due to the semiconductor film temporarily melting through the laser light irradiation; and therefore the semiconductor film located on the projective portion becomes thinner and unable to withstand stress.
Further, simulation results for temporal changes in a temperature distribution in a semiconductor film formed on an insulating film having unevenness during laser light irradiation to the semiconductor film are shown in FIGS. 23A to 23F. Line 8008 which is unevenness on a lower side of the graphs stands for a boundary between a base film formed of an oxide film and silicon. Further, an upper side line 8009 is a boundary between silicon and an air layer. And laser light is irradiated to the surface of the silicon which is line 8009. The oxide film thickness and the silicon film projective portion thickness are each 200 nm, and the gap between the concave and the convex is 1 μm. The laser light irradiation conditions were, with Gaussian, a peak energy density of 45,000 W/cm2 and σ=7×10−5 sec.
FIG. 23A shows a temperature distribution immediately after laser light irradiation, and FIGS. 23B to 23F show temperature distributions at 2.5 μsec intervals thereafter.
Regions shown by dark colors are portions that can be considered to have the highest temperature, and it can be seen that the dark colored portions become fewer as the state shifts from FIG. 23A to FIG. 23F. Regarding the temperature of the silicon which is between line 8008 and line 8009, it can be seen that, as time passes, portions on depressive portions of the base film shown by the line 8008 have temperature reduction before portions on projective portions of the base film 8008.
FIG. 24 shows simulation results for temporal changes in temperature due to location of a semiconductor film in irradiating laser light to the semiconductor film formed on an insulating film having unevenness.
A graph shown in FIG. 24 shows a semiconductor film temperature (K) on its vertical axis, and time (second) on its horizontal axis. A solid line shows the temperature of the semiconductor film located on a projective portion, and a dashed line shows the temperature of the semiconductor film located on a depressive portion. In the simulation shown in FIG. 24, temperature reduction temporarily stops along with a phase transformation at 1600 K, but after the phase transformation, the semiconductor film on the depressive portion which is shown by the dashed line begins dropping in temperature ahead of the semiconductor film on the projective portion, and it can be understood that the semiconductor film on the depressive portion undergoes earlier phase transformation.
The vicinity of the depressive portion a larger volume of the insulating film within a fixed range, so is has a larger heat capacity than the vicinity of the projective portion. That is because it is difficult for escaped heat to remain, and heat radiation is performed efficiently. Thus, after the semiconductor film melts due to the irradiation of laser light, in the process that heat within the semiconductor film is radiated to the insulating film to be solid, crystal nuclei tend to develop earlier in the vicinity of the depressive portion than the vicinity of the projective portion.
Crystal growth proceeds as time passes, from the crystal nuclei generated in the vicinity of the depressive portion toward the projective portion. One cause of grain boundaries developing on the projective portion can be considered to be the fact that crystal growths advancing from adjacent depressive portions come together on the projective portion between the two depressive portions.
Whatever the cause, the grain boundaries are selectively formed in upper portions of the projective portions while the grain boundaries tend not to be formed in portions located in the depressive portions on the semiconductor films thus crystallized.
From the semiconductor film crystallized by laser light, the applicants of the present invention considered using the portions formed on the depressive portions as a channel formation region of TFT.
Continuous wave laser light is most preferably used as the laser light, but pulse wave laser light may also be used. Note that it is preferable for a cross section of the projective portion in a direction orthogonal to the laser light scanning direction to be a quadrilateral shape, including a rectangular shape, or a triangular shape.
In accordance with the aforementioned structure, grain boundaries are selectively formed in the semiconductor films located on the projective portions of the insulating film when performing crystallization through laser light irradiation. Note that, although the semiconductor films located on the depressive portions of the insulating film have superior crystallinity since it is relatively difficult for grain boundaries to be formed therein, they do not always contain zero grain boundaries. However, it can be said that, even if grain boundaries do exist, crystal grains are larger, and the crystallinity is relatively superior, compared to the semiconductor films located on the projective portions of the insulating film. The locations at which grain boundaries are formed in the semiconductor films can therefore be predicted to a certain extent at the insulating film shape design stage. That is, the locations at which grain boundaries are formed can be set selectively with the present invention, and therefore it becomes possible to lay out the active layer such that, as much as possible, grain boundaries are not contained in the active layer, more preferably in channel formation regions.
The formation of grain boundaries in the TFT channel formation region can be avoided with the present invention by actively using the island-like semiconductor films located on the depressive portions of the insulating film as the TFT active layers. Substantial reduction in TFT mobility, reduction in ON current, and increase in OFF current due to the grain boundaries can be avoided.
Furthermore, the crystallinity of the active layer is enhanced and therefore a desired ON current value is obtained even when the active layer is small in size. Accordingly, the area of the whole circuit can be made small as well as the size of the semiconductor device.
When a semiconductor film crystallized by the above method is used in an integrated circuit which is one of semiconductor devices, there are some design restrictions, which are listed below.    1: The laser light scanning direction has to match the direction in which carriers move in a channel formation region of each TFT (channel length direction).    2: Laser light edges should not overlap an active layer of each TFT.    3: An active layer or a channel formation region has to be placed in a depressive portion of the base film.
To reduce complexity of the integrated circuit layout while abiding by the above three restrictions, the present invention uses the following designing method in manufacture of an integrated circuit.
The first step in designing an integrated circuit according to the present invention is to figure out what kinds of logic elements (hereinafter referred to as cells) constitute the integrated circuit and how many are used at the stage of logical calculus.
Then layout of a mask in each cell is determined. At this point, every mask has to be placed in the same direction in order to make the channel length direction of every TFT match the laser light scanning direction. Furthermore, the mask has to be placed such that an active layer or channel formation region of each TFT in each cell is formed from a semiconductor film that is on a depressive portion of a base film. By giving every TFT the same channel length direction, characteristic fluctuation between TFTs in the same cell can be avoided. Moreover, the use of a semiconductor film that is on a depressive portion prevents grain boundaries from forming in a channel formation region of a TFT. This makes it possible to avoid a significant drop in TFT mobility, reduction of ON current, and increase in OFF current which are caused by grain boundaries.
Then the only thing left to complete the objective integrated circuit layout is to combine a desired number of cells from various kinds of cells whose layout has already been determined and decide electrical connection between the cells. The various kinds of cells that constitute the integrated circuit are arranged so as to form stripe pattern columns along the longitudinal direction of the concave and convex of the base film, or the laser light scanning direction. In this specification, a group of cells arranged into a column is hereinafter referred to as cell column and the direction in which the cells are lined up will be called a cell column direction. Accordingly, the cell column direction is set equal to the longitudinal direction of the concave and convex of the base film and the laser light scanning direction. In addition, the channel length direction in each cell has to match the longitudinal direction of the concave and convex of the base film, the laser light scanning direction, and the cell column direction. These are important points in arranging the cells.
For every cell in the same cell column, the width in the direction perpendicular to the cell column direction (hereinafter referred to as cell width) should not exceed a certain range. A common voltage may be supplied from a power supply to every cell in the same cell column or different voltages may be supplied.
It is important to design the integrated circuit such that the cell width is smaller than the width of laser light in the direction perpendicular to the scanning direction. Strictly speaking, of a region irradiated with laser light, a region having a uniform energy density has to have a width in the direction perpendicular to the scanning direction that is smaller than the cell width.
Generally, laser light is lower in energy density near its edges than around its center. Accordingly, a semiconductor film crystallized by edges of laser light has poor crystallinity compared to one crystallized by the center of laser light. When running laser light over a semiconductor film, edges of laser light track therefore should not overlap a portion of the semiconductor film that later becomes a channel formation region of a TFT, more desirably, an active layer of the TFT. By designing the cell width smaller than the width of a laser light region having a uniform energy density in the direction perpendicular to the scanning direction, the semiconductor film crystallinity can be uniform throughout a cell and among cells and characteristic fluctuation between TFTs is avoided.
The cell width can be set wider when the width of a region having a uniform energy density in the direction perpendicular to the scanning direction is wider. A larger cell width means fewer restrictions in layout of TFTs in a cell and less complexity in design. Therefore it is desirable to use laser light that is rectangular or linear in section.
After layout is determined for all the cells, wires for electrically connecting the cells with one another are arranged between cell columns. When crystallizing the semiconductor film with laser light, laser light edges can overlap regions where the wires are placed without causing any problem. This is because the portions of the semiconductor film in these regions are removed in a later step and are not used as circuit elements.
With the above structure, laser light edges (seams) are readily prevented from overlapping cells and complex layout can be avoided. Moreover, no laser light is wasted on irradiating a region where no cell column is formed because regions having cell columns alone are irradiated with laser light. In other words, the above structure makes it easy to run laser light only over necessary regions for the minimum degree of crystallization. As a result, time required for laser light irradiation is shortened and the substrate processing speed is improved.
In this way, a desired number of cells from various kinds of cells whose layout has already been determined are combined to form cell columns and connection between cells is determined to build an objective integrated circuit. Thus design complexity of an integrated circuit can be reduced and the circuit layout is obtained efficiently while abiding by the above restrictions 1 through 3.
In addition, portions of the laser light which have a low energy density may also be blocked through a slit. Laser light having a relatively uniform energy density can be irradiated to the cell columns by using the slit, and crystallization can be performed uniformly. Further, the width of the laser light can be partially changed in accordance with the cell width by forming the slit, and in addition, restrictions on the layout of channel formation regions, and the layout of TFT active layers can be reduced. Note that the term laser light width refers to the length of the laser light in a direction orthogonal to the scanning direction.
Further, one laser beam obtained by synthesizing laser light emitted from a plurality of laser oscillation devices may also be used in laser crystallization. Portions having weak energy density in each of the laser lights can be mutually compensated for by using the above structure thereby obtaining a linear or a rectangular laser beam.
Further, the semiconductor film may also be crystallized by performing the irradiation of laser light without exposure to the atmosphere (for example, by using a specific gas atmosphere such as an inert gas, nitrogen, or oxygen, or a reduced pressure atmosphere) after film formation of the semiconductor film. Contaminants at the molecular level within a clean room, for example, boron contained within a filter used for increasing the cleanliness of air can be prevented from contaminating the semiconductor film when performing crystallization with laser light under the aforementioned structure.
If a flexible substrate is used to manufacture a semiconductor device, stress applied to a base film upon an increase in curvature of the substrate can be dispersed to a certain degree by matching the longitudinal direction of a projective portion of the base film with the direction of bus of the bent substrate.